(1) Field of the Invention
The present invention relates to an optical item, for example an ophthalmic lens, comprising a transparent substrate made from a synthetic resin or mineral glass, particularly with a high refractive index (nD25=1.5 or higher, preferably 1.55 or higher), at least one transparent coating such as an anti-abrasion coating or a primer layer and an anti-abrasion coating and a quarter waveplate layer interposed between the substrate and the transparent coating.
Conventionally, one or more coatings are formed on the main sides of a transparent substrate made of synthetic resin or mineral glass, such as an opthalmic lens, in order to impart to the item several advantageous properties such as shock resistance, abrasion resistance, reflection removal, etc.
Thus, generally, at least one side of the substrate is coated either directly with an abrasion resistant layer, or with a primer layer, generally a layer for improving the shock resistance of the lens, on which an abrasion resistant layer may be applied, the primer layer improving the anchoring of such an abrasion resistant layer on the substrate side. Finally, an anti-reflection coating may be applied to the abrasion resistant layer.
Generally, the substrate and the abrasion resistant layer or the primer layer have different refractive indices and consequently interference fringes occur because of such an index difference at the interface between the substrate and the abrasion resistant layer or the primer layer.
(2) Description of Related Art
U.S. Pat. No. 4,609,267 discloses a lens comprising a substrate made of a synthetic resin with a high refractive index (≧1.55), one face of which is coated with an abrasion resistant layer of a dielectric substance with a refractive index different from that of the substrate and which, in order to reduce the reflection at the interface between the substrate and the abrasion resistant layer, comprises at least one anti-reflective layer of a dielectric or metallic substance applied between the substrate and the abrasion resistant layer.
The abrasion resistant layer is a layer of SiO2.
The interposed anti-reflective layer is a quarter waveplate and consists either in a single layer of a blend of SiO2 and aluminum oxide or two layers, one layer of SiO2 and a second one made of a material selected amongst ZrO2, HfO2, Ti2O3, TiO2, Ta2O5, Si3N4, Yb2O3, Y2O3 or Al2O3.
Such anti-reflective and abrasion resistant layers are made through vacuum evaporation.
Nowadays, in order to form the abrasion resistant primer and coating layers, varnishes are used, i.e., compositions leading to a largely organic material as opposed to layers with an essentially mineral nature such as metal oxide and/or silicon oxide layers.
Moreover, in the industrial ophthalmic lens manufacturing processes, applying such varnish layers occurs by dipping in a varnish solution or dispersion bath or through centrifuging a solution or a dispersion on one side of the substrate.
If the refractive index of the varnish and that of the organic glass substrate do not match, i.e. if such refractive indices are significantly different, an interference fringe phenomenon also appears at the interface between the substrate and the varnish.